Three-dimensional Stereotactic surface projection (3D SSP) is a proven method of extracting cortical activity and mapping it onto the brain surface. Fluorodeoxyglucose (FDG) is a PET imaging compound, labeled with the radioactive isotope [18F]. For brain imaging, FDG issued to image cerebral metabolic rate of glucose consumption. 3D SSP has been shown to be a successful method, e.g., for detecting image patterns associated with Alzheimer's disease using FDG in PET brain imaging (PET FDG). 3D SSP has also been used for other PET and SPECT tracers, for example brain perfusion studies with SPECT using Ceretec. The 3D SSP method defines a large number of surface points on a brain model in standard space. Each surface point is associated with a normal vector. With reference to FIG. 1, when analyzing a patient PET or SPECT scan, the scan is first spatially normalized to the standard space. Then, for each cortical surface point, the data is data is sampled along the opposite direction of the normal vector into the brain to a certain depth, as shown in FIGS. 2A and 2B. The value of the maximum voxel along the ray into the brain is then mapped back to the surface point. The original 3D SSP method extends a fixed depth into the brain from each surface voxel. (Minoshima et al., A diagnostic approach in Alzheimer's disease using three-dimensional stereotactic surface projections of fluorine-18-FDG PET. J Nucl Med. 1995 July; 36(7):1238-48)
[18F]Flutemetamol is another PET imaging compound which binds to amyloid plaques in the brain and hence show deposition of amyloid in gray matter. Both FDG and Flutemetamol show changes in the cortical regions related to Alzheimer's disease. Yet, while PET FDG exhibits the highest uptake in the gray matter, [18F]Flutemetamol can exhibit high uptake in both gray and white matter. In [18F]Flutemetamol imaging of amyloid, there will be a significant uptake in gray matter if amyloid is present, while it will be very little or none if no amyloid is present. However, [18F]Flutemetamol also undergoes significant non-specific uptake in white matter for both amyloid positive and amyloid negative scans. This means that when using SSP in images with no or little amyloid, ie, a negative scan, there is a risk that the standard method will pick up high intensity white matter regions.
3D SSP methods of the prior art have extended the same depth into the brain from every surface voxel. Using such a single-depth for the analysis, the method thus can inadvertently extend into white matter, which will exhibit high uptake of an imaging agent like [18F]Flutemetamol. Because SSP is an attractive method for imaging both compounds, there is therefore a need in the art for a method and system which can use 3D SSP for amyloid imaging agents to observe uptake in gray matter but simultaneously minimizing the risk for blending in signal coming from white matter, as both tissues may exhibit high uptake.